1. Field of the Invention
The present invention relates to a process for producing a high purity aromatic polycarboxylic acid useful as a material for polyesters, polyamides, polyimides and liquid crystalline polymers and, more particularly, to a process for producing a high purity naphthalenepolycarboxylic acid and a high purity biphenylpolycarboxylic acid which are not easily purified, heretofore.
2. Description of the Prior Art
Aromatic polycarboxylic acids are commercially important substances as chemical intermediates and have a wide demand as materials of polyesters, polyamides, polyimides and liquid crystal polymers which are used for fibers, bottles, films and electronic applications.
The aromatic polycarboxylic acids industrially used in a wide range of applications are, for example, terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, pyromellitic acid, 2,6-naphthalenedicarboxylic acid, 4,4xe2x80x2-biphenyldicarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid and 3,3xe2x80x24,4xe2x80x2biphenyltetracarboxylic acid.
As the process for producing an aromatic polycarboxylic acid, processes in which an aromatic polyalkylhydrocarbon such as xylene, a dialkylnaphthalene, a dialkylbiphenyl, a tetraalkylnaphthalene and a tetraalkylbiphenyl is oxidized with molecular oxygen at a high temperature and a high pressure in the presence of a heavy metal such as Co and Mn and a bromine compound in acetic acid as the solvent or oxidized with the air in the presence of nitric acid or chromic acid, are known. The aromatic polycarboxylic acid obtained by the oxidation reaction contains impurities such as monocarboxylic acids and aldehyde which are intermediate products of the oxidation reaction, addition products of bromine and metal components which are derived from the catalyst and coloring substances having unknown structures.
Recently, necessity for recycling plastic materials such as polyesters is increasing and recycle and reuse of materials are conducted, for example, by decomposition of PET bottles. However, in general, aromatic polycarboxylic acids obtained by the above decomposition contain impurities such as colored components and foreign substances.
When the aromatic polycarboxylic acids containing impurities are used as materials in the polymerization with diols and diamines, the obtained resins exhibit inferior physical and mechanical properties such as inferior heat resistance, mechanical strength and dimensional stability. Therefore, such aromatic polycarboxylic acids cannot be used as the materials for polyesters, polyamides and polyimides. In general, crude aromatic polycarboxylic acids obtained by oxidation are colored yellow or black and cannot be used for applications requiring transparency such as bottles and films without further treatments. Moreover, since the crude aromatic polycarboxylic acids are in the form of particles having small diameters, handling is not easy and problems tend to arise during polymerization. Therefore, the process for producing a high purity aromatic polycarboxylic acids having improved hue and particle diameters industrially advantageously has been studied for a long time.
In general, purification of an organic compound is conducted in accordance with distillation, crystallization, adsorption or a combination of these operations. However, since aromatic polycarboxylic acids have a temperature of self-decomposition lower than the boiling point, the purification by distillation is substantially impossible. Moreover, since aromatic polyearboxylic acids have poor solubility in solvents ordinarily used in industrial production, the purification by crystallization cannot be easily conducted. In particular, naphthalenepolycarboxylic acid and biphenylpolycarboxylic acids have very poor solubility in various solvents and no industrially advantageous processes for producing a high purity naphthalenepolycarboxylic acid or a high purity biphenylpolycarboxylic acid have been established.
Many processes in which a salt is formed from an amine and an aromatic polycarboxylic acid and the purification is conducted in accordance with a treatment such as crystallization and adsorption, have Been proposed. For example, in Japanese Patent Application Laid-Open No. Heisei 7(1995)-118200, a process in which a salt of an amine and naphthalenedicarboxylic acid is purified by crystallization from a mixed solvent of water and an alcohol, is disclosed. In Japanese Patent Application Laid-Open No. Heisei 10(1998)-53557, a process in which an amine salt of naphthalenedicarboxylic acid is purified by crystallization in a mixed solvent of water and an aliphatic or alicyclic ketone, is disclosed.
The amine salt of an aromatic polycarboxylic acid thus obtained is decomposed by heating and the aromatic polycarboxylic acid is crystallized. As the process for this treatment, for example, processes in which an amine salt of naphthalenedicarboxylic acid is decomposed by blowing steam at 140xc2x0 C. into an aqueous solution of the amine salt at 130xc2x0 C. or by flushing an aqueous solution of the amine salt with boiling under an ordinary pressure from a heated and pressurized tank are disclosed in Japanese Patent Application Laid-Open No. Showa 50(1975)-142542. Processes in which an amine salt of terephthalic acid is decomposed by heating an aqueous solution of the amine salt under the refluxing condition at an ordinary pressure or by blowing steam at 135xc2x0 C. into an aqueous solution of the amine salt are disclosed in Japanese Patent Application Laid-Open No. Heisei 6(1994)-293696.
The above processes for producing an aromatic polycarboxylic acid is not industrially advantageous due to the following reasons:
(1) In the process in which an aqueous solution is flushed with boiling under an ordinary pressure from a heated and pressurized tank and the process in which an aqueous solution is heated under the refluxing condition under an ordinary pressure, the rate of decomposition is small due to a low temperature for the decomposition and the processes are economically disadvantageous since a great amount of water must be vaporized.
(2) In the process in which an amine salt is decomposed by blowing steam into an aqueous solution of the amine salt, a great amount of steam is required to obtain an aromatic polycarboxylic acid in a high yield since the concentration of the amine salt in the aqueous solution decreases and the process is not economically advantageous.
Decomposition of an amine salt of an aromatic polycarboxylic acid was conducted by the present inventors in accordance with the industrially most popular process in which heat is supplied from the jacket of a salt decomposition reactor. It was found that the decomposition of the amine salt of an aromatic polycarboxylic acid could not be continued for a long time with stability since crystals of the amine salt of an aromatic polycarboxylic acid were formed, tightly stuck to the wall surface of the salt decomposition reactor and covered the surface of heat transfer.
In the process in which the salt of an aromatic polycarboxylic acid and an amine is purified and the purified amine salt is decomposed, the utility cost is great since great amounts of steam and heating gas are required. When the concentration of the slurry is increased to decrease the amount of energy consumption, a high purity aromatic polycarboxylic acid in the form of particles having large diameters cannot be obtained.
The present invention has an object of providing a process for industrially producing a high purity aromatic polycarboxylic acid having excellent hue and large particle diameters at a low cost of production for a long time with stability in accordance with a process having a simple construction by decomposing an amine salt of an aromatic polycarboxylic acid obtained from a crude aromatic polycarboxylic acid.
As the result of extensive studies by the present inventors to achieve the above object, it was found that a high quality aromatic polycarboxylic acid can be easily produced in accordance with a process having a simple construction and the consumption of energy can be decreased remarkably when a salt of a crude aromatic polycarboxylic acid and an amine is formed and an aqueous solution of the amine salt of the aromatic polycarboxylic acid is continuously decomposed by heating.
As the first invention, the present invention provides a process for producing a high purity aromatic polycarboxylic acid by purifying a crude aromatic polycarboxylic acid in accordance with following steps:
(I) a step of forming a salt which comprises forming an amine salt of an aromatic polycarboxylic acid from the crude polycarboxylic acid and an amine;
(II) a step of decomposing a salt which comprises continuously conducting in a salt decomposition reactor three operations of (i) supplying an aqueous solution of the amine salt of an organic polycarboxylic acid formed in the step of forming a salt, (ii) removing the amine and water by distillation under heating and (iii) taking out a slurry containing crystallized aromatic polycarboxylic acid; and
(III) a step of separating crystals which comprises separating and recovering crystals of the aromatic polycarboxylic acid from the slurry which is taken out in the step of decomposing a salt.
It was also found by the present inventors that an amine salt of an aromatic polycarboxylic acid could be decomposed with stability for a long time in accordance with a process having a simple construction without sticking of the aromatic polycarboxylic acid to the wall surfaces of the heat exchanger or the salt decomposition reactor when a slurry in the salt decomposition reactor which comprised an aqueous solution of the amine salt of an aromatic polycarboxylic acid and the aromatic polycarboxylic acid formed by the decomposition was circulated through a heat exchanger and heated, the pressure of the slurry in the heat exchanger was kept at the vapor pressure of the slurry or higher, the slurry was introduced into the salt decomposition reactor, the amine salt was decomposed by removing a mixture of the amine and water by distillation and the aromatic polycarboxylic acid was crystallized.
As the second invention, the present invention provides a process for producing a high purity aromatic polycarboxylic acid by purifying a crude aromatic polycarboxylic acid in accordance with (I) a step of forming a salt which comprises forming an amine salt of an aromatic polycarboxylic acid from the crude polycarboxylic acid and an amine, (II) a step of decomposing a salt which comprises (i) conducting in a salt decomposition reactor operations of supplying an aqueous solution of the amine salt of an aromatic polycarboxylic acid formed in the step of forming a salt, (ii) removing the amine and water by distillation under heating and (iii) taking out a slurry containing crystallized aromatic polycarboxylic acid, and (III) a step of separating crystals which comprises separating and recovering crystals of the aromatic polycarboxylic acid from the slurry which is taken out in the step of decomposing a salt, wherein
in the step of decomposing a salt, the slurry in the salt decomposition reactor which comprises the aqueous solution of the amine salt and the aromatic polycarboxylic acid formed by the decomposition is circulated through a heat exchanger, heat is supplied in the heat exchanger while a pressure of the slurry is kept at a vapor pressure of the slurry or higher, a mixture of the amine and water is removed by distillation in the salt decomposition reactor, and the aromatic polycarboxylic acid is crystallized in the salt decomposition reactor.